As HIV has evolves to evade the particulars of humans’ immune systems, it may be becoming less virulent, at least in Botswana, NPR reports. Publishing their findings in the Proceedings of the National Academy of Sciences, researchers sequenced viral genes from about 2,000 women in Botswana and South Africa to compare how the ever-mutating virus evolved over time. Because the HIV epidemic began about a decade earlier in Botswana than in South Africa, this difference permitted the researchers to make inferences about how the passage of time is affecting the virus’s evolution.

The researchers found that the HIV in the women from Botswana replicated about 10 percent more slowly than the HIV in the South African women. This difference likely shifts the average time that untreated HIV would take to progress to AIDS from 10 years to 12.5 years.